Re: [RFC PATCH v2] sched: reflect sched_entity movement into task_group's utilization

[Vincent Guittot <vincent.guittot@linaro.org>]

On 1 June 2016 at 15:36, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, May 24, 2016 at 10:57:32AM +0200, Vincent Guittot wrote:
>
>> +/*
>> + * Save how much utilization has just been added/removed on cfs rq so we can
>> + * propagate it across the whole tg tree
>> + */
>> +static void set_tg_cfs_rq_util(struct cfs_rq *cfs_rq, int delta)
>> +{
>> +     if (cfs_rq->tg == &root_task_group)
>> +             return;
>> +
>> +     cfs_rq->diff_util_avg += delta;
>> +}
>
> function name doesn't really reflect its purpose..

ok, i will change it

>
>> +
>> +/* Take into account the change of the utilization of a child task group */
>
> This comment could be so much better... :-)

yes, i'm going to add more details of what is done

>
>> +static void update_tg_cfs_util(struct sched_entity *se, int blocked)
>> +{
>> +     int delta;
>> +     struct cfs_rq *cfs_rq;
>> +     long update_util_avg;
>> +     long last_update_time;
>> +     long old_util_avg;
>> +
>> +
>> +     /*
>> +      * update_blocked_average will call this function for root cfs_rq
>> +      * whose se is null. In this case just return
>> +      */
>> +     if (!se)
>> +             return;
>> +
>> +     if (entity_is_task(se))
>> +             return 0;
>> +
>> +     /* Get sched_entity of cfs rq */
>> +     cfs_rq = group_cfs_rq(se);
>> +
>> +     update_util_avg = cfs_rq->diff_util_avg;
>> +
>> +     if (!update_util_avg)
>> +             return 0;
>> +
>> +     /* Clear pending changes */
>> +     cfs_rq->diff_util_avg = 0;
>> +
>> +     /* Add changes in sched_entity utilizaton */
>> +     old_util_avg = se->avg.util_avg;
>> +     se->avg.util_avg = max_t(long, se->avg.util_avg + update_util_avg, 0);
>> +     se->avg.util_sum = se->avg.util_avg * LOAD_AVG_MAX;
>> +
>> +     /* Get parent cfs_rq */
>> +     cfs_rq = cfs_rq_of(se);
>> +
>> +     if (blocked) {
>> +             /*
>> +              * blocked utilization has to be synchronized with its parent
>> +              * cfs_rq's timestamp
>> +              */
>> +             last_update_time = cfs_rq_last_update_time(cfs_rq);
>> +
>> +             __update_load_avg(last_update_time, cpu_of(rq_of(cfs_rq)),
>> +                       &se->avg,
>> +                       se->on_rq * scale_load_down(se->load.weight),
>> +                       cfs_rq->curr == se, NULL);
>> +     }
>> +
>> +     delta = se->avg.util_avg - old_util_avg;
>> +
>> +     cfs_rq->avg.util_avg =  max_t(long, cfs_rq->avg.util_avg + delta, 0);
>> +     cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * LOAD_AVG_MAX;
>> +
>> +     set_tg_cfs_rq_util(cfs_rq, delta);
>> +}
>
> So if I read this right it computes the utilization delta for group se's
> and propagates it into the corresponding parent group cfs_rq ?

yes

>
>> @@ -6276,6 +6368,8 @@ static void update_blocked_averages(int cpu)
>>
>>               if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true))
>>                       update_tg_load_avg(cfs_rq, 0);
>> +             /* Propagate pending util changes to the parent */
>> +             update_tg_cfs_util(cfs_rq->tg->se[cpu], true);
>
> And this is why you need the strict bottom-up thing fixed..

yes

>
>> @@ -8370,6 +8464,20 @@ static void detach_task_cfs_rq(struct task_struct *p)
>>
>>       /* Catch up with the cfs_rq and remove our load when we leave */
>>       detach_entity_load_avg(cfs_rq, se);
>> +
>> +     /*
>> +      * Propagate the detach across the tg tree to ake it visible to the
>> +      * root
>> +      */
>> +     for_each_sched_entity(se) {
>> +             cfs_rq = cfs_rq_of(se);
>> +
>> +             if (cfs_rq_throttled(cfs_rq))
>> +                     break;
>> +
>> +             update_load_avg(se, 1);
>> +             update_tg_cfs_util(se, false);
>> +     }
>>  }
>>
>>  static void attach_task_cfs_rq(struct task_struct *p)
>> @@ -8399,8 +8507,21 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
>>
>>  static void switched_to_fair(struct rq *rq, struct task_struct *p)
>>  {
>> +     struct sched_entity *se = &p->se;
>> +     struct cfs_rq *cfs_rq;
>> +
>>       attach_task_cfs_rq(p);
>>
>> +     for_each_sched_entity(se) {
>> +             cfs_rq = cfs_rq_of(se);
>> +
>> +             if (cfs_rq_throttled(cfs_rq))
>> +                     break;
>> +
>> +             update_load_avg(se, 1);
>> +             update_tg_cfs_util(se, false);
>> +     }
>> +
>>       if (task_on_rq_queued(p)) {
>>               /*
>>                * We were most likely switched from sched_rt, so
>
> So I would expect this to be in attach_task_cfs_rq() to mirror the
> detach_task_cfs_rq().

yes. My goal what to rely on the enqueue to the propagate the move
during a task_move_group but it's better to place it in
attach_task_cfs_rq so we have same sequence for attaching and
detaching

>
> Also, this change is somewhat independent but required for the 'flat'
> util measure, right?

don't know if it's needed for flat util measure as the main interest
of flat util measure is to not go through the tg tree